1. Field of the Invention
The present invention generally relates to an image forming apparatus, such as a color copier, or a color printer, using plural photoconductor drums to form color images with four colors including cyan, magenta, yellow, and black.
2. Description of the Related Art
Conventionally, many image forming apparatuses employ a multiple drum system for forming color images. In forming color images with the multiple drum system, first, images of different color are formed on respective drums corresponding to each of the colors, and then, images for each color are separately transferred in an overlapped manner onto a transfer sheet placed on a transfer belt.
An image forming apparatus using the multiple drum system has a benefit of forming images at high speed. The image forming apparatus, however, has difficulty in controlling color misalignment. Sources causing the color misalignment are, for example, skew difference, registration difference in a sub-scanning direction, magnification error in a main scanning direction, and registration difference in a main scanning direction. Such color misalignment is a cause for lowering the quality of output images.
Japanese Laid-Open Publication No. 11-84803 discloses an art for controlling the color misalignment created upon forming color images. This art is able to control color misalignment caused by registration difference with regard to a scanning line curve of a line pattern.
More particularly, with this art, each line pattern image including the scanning line curve, which is formed on each photoconductor and then transferred to a transfer medium, can be detected by disposing three or more detection points situated in a main scanning direction. Accordingly, a value for correcting registration difference in the main scanning direction is calculated in accordance with the detected line pattern image.
Meanwhile, using the appropriate detected data for calculating the amount for correcting the color misalignment sources is essential for sufficiently correcting the color misalignment. That is, in correction of the color misalignment, it is important to determine which of the detected data (detected by plural detection sensors) should be employed for the correction. If the detected data is not used appropriately, the correction of the color misalignment will be insufficient.